trigger ClosedOpportunityTrigger on Opportunity (after insert, after update) {
    List<Task> taskList = new List<Task>();
    for(Opportunity oppo : Trigger.New) {
        if(oppo.StageName == 'Closed Won') {
            taskList.add(new Task(Subject='Follow Up Test Task',WhatId=oppo.ID));
        }
    }
    insert taskList;
}